1. Field of the Invention
This invention relates to a nitride layer forming method, a magnetic circuit forming member, an armature, a wire dot printer head and a wire dot printer.
2. Discussion of the Background
The wire head printer head is adapted to rock an armature to which a printing wire is connected between a print position and a standby position and impact the tip of the wire in a print medium when the armature is rocked to the print position to perform printing. As the wire dot printer head, a device has been proposed, in which magnetic flux is generated in the periphery of the armature as a rocking object by a coil, and a magnetic circuit for attracting the armature from the standby position to the print position is generated to perform printing (See JP-A-2000-280497). In the invention described in JP-A-2000-280497, in order to heighten the strength of the armature, a nitride layer is formed on the surface of the armature.
On the other hand, the armature is provided with a fulcrum shaft functioning as the center of rotation and a through hole for inserting the fulcrum shaft, and disposed to be freely rotated (rocked) by holding the fulcrum shaft. In this case, the fulcrum shaft is rotatably inserted in the through hole. Consequently, during the printing operation, the fulcrum shaft is slid along the through hole, so that the inner surface of the through hole wears away. In order to prevent such abrasion, generally a nitride layer is formed on the surface (including the inner surface of the through hole) of the armature to improve the strength of the armature.
In the armature having the nitride layer formed on the whole surface thereof, however, the magnetic flux characteristic is deteriorated by the nitride layer. In recent years, it is necessary to increase the printing speed, that is, rock the armature between the print position and the standby position at a high frequency as much as 2500 times/sec. In that case, the armature having the nitride layer formed on the whole surface has the problem that the magnetic characteristic required for high-speed printing can not be obtained, so high-speed printing can not be performed.
Further, in the case where the fulcrum shaft is provided in the through hole as in the above armature, it is necessary to form a nitride layer or the like on the inner surface of the through hole. In the case where a nitride layer is not formed on the inner surface of the through hole, the fulcrum shaft slides along the through hole during the printing operation, so that the inner surface of the through hole wears away to cause jogging of the fulcrum shaft, resulting in the problem that the rocking operation of the armature becomes unstable.